Long period gratings (LPGs), like fiber Bragg gratings (FBGs), are periodic modulations of the refractive index of the core of an optical waveguide—typically a single-mode optical fiber—but the LPG has a much longer period (typically 10 μm to 1 mm) compared to the Bragg grating (<1 μm). LPGs couple light from the mode propagating along the fiber core to modes associated with co-propagating cladding modes of the fiber. Due to the high losses typically experienced by cladding modes, the LPG behaves as a notch filter. Thus, unlike FBGs which reflect wavelengths selected by the periodicity of the grating back along the core of the optical fiber, LPGs act as notch filters with low back reflection. Depending on the regularity of the grating period and the length of the grating, the band rejection of an LPG can have a width of typically 30 nm and the loss at the peak can approach −30 dB (James et al. 2003).
For a LPG, the wavelengths of the core mode that couples into the cladding modes are characterized by the phase-matching conditionλf=Λ[neffcore(λ, n1, n2)−neffcladdingi(λ, n2, n3)]  (equation 1)where neff,core is the effective core refractive index which is a function of wavelength, core refractive index n1 and cladding refractive index n2, and neff,cladding is the effective cladding refractive index of the ith mode, which is a function of wavelength, cladding refractive index n2 and surrounding refractive index n3, and Λ is the period of the LPG. From this expression it is apparent that any physical, mechanical, or environmental parameter that is capable of changing the effective refractive indices differentially (e.g., refractive index of the surrounding medium) or of changing the period of the grating (e.g., mechanical strain), will lead to a change in the attenuation spectrum of the LPG. Given proper calibration, one can then use the shift in the attenuation lines to interrogate the environmental parameter (see James et al., 2003). This measurement principle has been discussed in a number of publications, and has led to considerable interest in using LPGs as inexpensive, robust and sensitive sensors (Allsop et al. 2001; Bhatia et al. 1996; Bhatia 1999; Chong et al. 2004; DeLisa et al. 2000, Grubsky et al. 2000; James et al. 2003; Khaliq et al. 2001; Khaliq et al. 2002; Lee et al. 2003; Shu et al. 1999).
When using LPGs as chemical sensors the differential change in refractive index is typically induced by a changing refractive index of the medium surrounding the cladding. Since the evanescent wave of the cladding modes effectively “probes” the medium outside the fiber, its refractive index will also influence the effective refractive index of the cladding. If, for example, in an extreme case the refractive indices of cladding and surrounding medium are identical, the cladding loses its ability to guide light and will essentially be extended infinitely. LPGs can hence be used as sensitive sensors for changes in refractive indices (Bhatia 1999; Chong et al. 2004; Lee et al. 2003; Shu et al. 1999). For example, aromatic compounds in a hydrocarbon matrix have been detected by changes in an LPG spectrum (Allsop et al. 2001), producing a wavelength change of ˜0.4 nm for a concentration of xylene of 0.5% (vol) in a paraffin solution. The detection limit was reported as 0.04% (˜400 ppm).
One approach to the use of LPGs as sensors has been to coat the LPG with a reactive coating that undergoes a chemical and/or physical change when exposed to the analyte. For example, Luo et al. (2002) used LPGs coated with carboxymethylcellulose (CMC) to detect humidity, and LPGs coated with metal nanoclusters embedded in polyethylenimine (PEI) to detect sulfide-based chemical warfare agents. However, the CMC LPG sensors exhibited strong temperature dependence, and the PEI LPG sensors underwent an irreversible reaction with the sulfide, such that they were not re-usable. It is also likely that the PEI LPG sensors would cross-react with other sulfides/thiols, suggesting poor specificity to the target compound. Similarly, Murphy et al. (U.S. Pat. No. 5,864,641, issued Jan. 26, 1999) proposed a physically, electrically, and chemically reactive coatings for LPG sensors. Murphy et al. discussed such a coating populated with reactive sites for the binding and hence detection of protein, but did not demonstrate any specificity to a particular protein.
Clearly there are a number of obstacles, relating to factors such as temperature sensitivity, specificity to the target species, reversibility of interaction with the target species, and sensitivity with respect to refractive index, which must be overcome before LPGs may be used practically as sensors.